Since recent OS X release (10.9, 10.10) it is no longer possible to save a .txt file because the system require a .tab extension.
This *.tab extension was always a nightmare; on my systems a part FileMaker Pro and Bento not any application knows that it is text file tab separated; so the double-click on the file won't open correctly (it suggest Bento if you still have it).
Note : the script step with a file path specified works; this is only an issue when the user have to indicate the file.
With older OS, i think one enduser can force to use "txt" file extension but it is no longer possible.
A bug report was made and "vaguely" acknowledged by FMI: Can't export files with the .txt extension
But without any following even by philmodjunk.
4 year later it is still here so maybe a feature request would be a better way ?